Add some basic sanity tests to RootScroller for cases with remote frames.

Added one test that deals with a remote iframe and another with a remote main
frame. RootScroller doesn't quite work in OOPIF scenarios yet but it's still
behind a flag so that's ok. These are mostly testing that we don't crash in the
normal case and that scrolling works ok.

BUG=641775

Review-Url: https://codereview.chromium.org/2290173004
Cr-Commit-Position: refs/heads/master@{#416364}

third_party/WebKit/Source/web/tests/data/root-scroller-child.html

@@ -23,8 +23,8 @@
       }
 
       #spacer {
-        width: 400px;
-        height: 400px;
+        width: 800px;
+        height: 800px;
       }
     </style>
   </head>